Taguchi Optimization Approach for Production of

Activated Carbon from Phosphoric Acid Impregnated Palm

Kernel Shell by Microwave Heating

Anirban Kundu,a Bhaskar Sen Gupta,c M.A. Hashimb Ghufran Redzwan,a

1 aInstitute of Biological Sciences, University of Malaya, 50603, Kuala Lumpur, MalaysiabDepartment of Chemical Engineering, University of Malaya, 50603,Kula Lumpur, MalaysiacSchool of Planning, Architecture and Civil Engineering, Queen’s University Belfast, David Keir Building, Belfast, BT9 5AG, UK

1 Corresponding Author

Ghufran Redzwan, Institute of Biological Sciences, University of Malaya, 50603, Kuala Lumpur, Malaysia

Tel. No.   +60379676797

Fax No.  +60379674178

E-mail Address  ghufran@um.edu.my

Abstract

Taguchi method was applied to investigate the optimal operating

conditions in the preparation of activated carbon using palm

kernel shell with quadruple control factors: irradiation time,

microwave power, concentration of phosphoric acid as impregnation

substance and impregnation ratio between acid and palm kernel

shell. The best combination of the control factors was microwave

power of 800 W, irradiation time of 17 min, impregnation ratio of

2, and acid concentration of 85%. The noise factor (particle

size), considered in a separate outer array, had no effect on the

quality of the activated carbon as confirmed by t-test. Activated

carbon prepared at optimum combination of control factors had

high BET surface area of 1,473.55 m² g-1 and high porosity. The

adsorption equilibrium and kinetic data can satisfactorily be

described by the Langmuir isotherm and a pseudo-second-order

kinetic model, respectively. The maximum adsorbing capacity

suggested by the Langmuir model was 1000 mg g-1.

Key Words: Taguchi method; robust design; activated carbon;

phosphoric acid; microwave heating.

1. Introduction

Activated carbon (AC) has been used as an adsorbent since the

ancient past in Egypt and India for removal of unwanted odor,

taste, dyes, heavy metals, and organic substances from wastewater

(Marsh & Reinoso, 2006). However, it is generally accepted that

reduction of the production cost remains the main challenge for

mass scale use of activated carbon. In recent years, researchers

have used low cost materials (Rafatullah et al., 2010) to reduce

the cost of production, however, the huge consumption of energy

in a conventional muffle furnace during the preparation remains a

constraint to cost reduction. Heating in a muffle furnace

typically takes one to seven hours (Baccar et al., 2009; Dural et

al., 2011; Nabais et al., 2011; San Miguel et al., 2003; Tay et

al., 2009) to convert the raw material into porous activated

carbon consuming considerably high amount of energy. The reason

for requiring this long heating time may be due to the fact that

heating occurs from the surface of the material towards the core.

This makes the process time consuming and there is no guarantee

that the heating is uniform. Thus, a requirement for developing a

more effective heating method for the production of activated

carbon with optimal operation conditions is very important. This

can be achieved by reduction of the heating time, optimization of

the energy as well as chemical use. Microwave technology can

reduce the production time considerably as microwaves can

penetrate the raw material and consequently the heat is produced

due to molecular interaction with the microwave energy. In this

way heat is generated throughout the bulk of the material, thus

reducing the processing time and improving the overall quality of

the activated carbon (Thostenson & Chou, 1999). However, a robust

optimization study is required to optimize the process parameters

and reduce production cost.

Taguchi method for optimization was developed by Genichi Taguchi

to improve the quality of product with a unique set of

“orthogonal array” experiments which are balanced with respect to

all control factors and yet are minimum in number. This in turn

ensures minimum use of resources and brings down the production

cost. The ‘log’ functions of desired output defined by Taguchi

as Signal-to-Noise ratios (S/N), serves as the objective function

for data analysis and optimization (Apte, 2000). The most

appropriate design is selected on the basis of the best levels of

control factors having maximum S/N ratios. Taguchi method also

considers the effects of Noise factors which are inconvenient to

control. Aber et al., (2009) and Darabi et al., (2012) have used

Taguchi optimization method to produce activated carbon however

none of them considered the outer array for the noise factor.

In this study, palm kernel shell (PKS) was used as the raw

material as it is the most widely available agricultural waste

material in Malaysia. This raw material was impregnated with

phosphoric acid. In order to understand the physical character of

the produced activated carbon at the optimum condition, scanning

electron micrograph (SEM) imaging, Fourier transform infrared

spectroscopy (FTIR) and BET surface area were studied. Methylene

blue (MB) was used as the adsorbate. Both isotherm and kinetic

studies were performed for MB adsorption.

2. Materials and Method

2.1 Materials

Palm kernel shells were collected from a local palm oil mill near

Kuala Lumpur, Malaysia and washed to remove dust and dirt and

subsequently dried in an oven at 105⁰C until the mass reached a

constant value. Phosphoric acid and MB of analytical grade were

obtained from R&M chemicals, UK and Fluka Chemicals,

respectively. All solutions were prepared in distilled water. A

stock solution of MB of concentration 1000 mg L-1 was prepared

and the desired concentrations of the MB solutions were prepared

by dilution method with distilled water.

2.2 Taguchi experimental design

Taguchi method, design of experiments and multiple regression

analysis are some of the important tools used for robust design

to produce high quality products efficiently (Phadke, 2009).

Taguchi Method is based on testing the sensitivity of a set of

response variables to a set of control parameters (or independent

variables) by considering experiments in “orthogonal array” with

an aim to attain the optimum setting of the control parameters.

Orthogonal arrays provide a best set of well balanced (minimum)

experiments. The signal to noise ratios (S/N), which are log

functions of desired output, serve as the objective functions for

optimization, aid the data analysis and the prediction of the

optimum results. There are three forms of S/N ratio, such as

smaller-the-better, larger-the-better, and nominal-the-best that

are of common interest for optimization of static problems

expressed by Eqs. 1, 2 and 3, respectively.

n=−log [meanof∑ ofsquaresofmeasureddata ] (1)

n=−10log[meanof∑ofsquaresofreciprocalofmeasureddata ] (2)

n=−10log [squareofmeanvariances

]

(3)

The smaller-the-better S/N ratio is chosen for the problems for

which the ideal output should be zero. The larger-the-better S/N

ratio characterizes the better performance for the output. The

nominal-the-best S/N ratio considers the situation where a

specific value is most desired; any value larger or smaller is

not the desired output. In this study, the algorithm for larger-

the-better S/N ratio was chosen as the adsorption capacity of the

adsorbent should be as high as possible at the optimum condition

(Aber et al., 2009; Li et al., 2010). L16 orthogonal array was

chosen with four operational parameters known as control factors,

namely microwave power (W) in watt, time (T) in minutes,

impregnation ratio (IR) of amount of acid to amount of PKS, and

concentration of acid in %, with four levels for each as shown in

Table 1. Noise factor taken in this study is the particle size of

the raw material. The particle size of 0.5 mm to 1 mm is

considered as the noise factor 1 (NF1) and between 1mm to 2 mm is

taken as noise factor 2 (NF2). A separate outer array is

constructed to include the noise factors and it requires

replication of each run in the orthogonal array for each noise

condition. Table 2 shows the L16 orthogonal array including the

noise factors. Minitab 16 software was used to formulate the

Taguchi orthogonal array and also in the computation of ANOVA and

level averages.

<Table 1>

<Table 2>

2.3 Preparation of adsorbent

The PKS was grinded into two particle size range 1000-2000 µm and

500-1000 µm. The material was washed several times with distilled

water and dried. The ground palm kernel fibers were impregnated

with phosphoric acid at three different ratios as the Taguchi

design matrix suggested. Forty grams of the PKS was mixed with

requisite amount of phosphoric acid for four hours with constant

stirring at 120 rpm at 27 oC. The slurry was then dried in a

vacuum oven at 100 oC for 24 hours. The resultant sample loaded

with the chemical was placed vertically in a specially designed

quartz tube in a microwave oven with frequency 2450 MHz

(SYNOTHERM corporation, model HAMiLab-C). The microwave power and

irradiation time were set at the values assigned for the specific

experimental run. The microwave was preheated at 100 W for five

minutes to heat up the magnetron inside the instrument. The tube

was purged with nitrogen gas at a flow of 0.5 L m-1 for five

minutes before microwave treatment to purge air. This flow rate

was maintained during the activation and cooling stages. The

activated carbon obtained was then washed thoroughly with

distilled water until the pH of the washing solution became

constant.

2.4 Physical and chemical characterizations of adsorbent

The pore structures of the prepared activated carbons were

analyzed using N2 adsorption and SEM. Nitrogen

adsorption/desorption isotherms were used to measure the BET

surface area, total pore volume and density functional theory

(DFT) pore size distribution at 77K by Quantachrome Autosorb-6B.

field emission scanning electron microscopy (FESEM) (Brand Zeiss

Model Auriga) was applied to study the surface morphology and

pore development. Fourier Transform Infrared (FTIR) spectroscopy

was used to analyze the surface functional groups of the

precursor material and the prepared activated carbons with the

help of FTIR spectroscope (Bruker, IFS66v/S). Spectra are

recorded at a range of 400 to 4000 cm−1.

2.5 Adsorption experiments

Batch adsorption experiments were conducted to eliminate volume

correction caused by liquid displacement. Experiments were

performed in 250 mL Erlenmeyer flasks containing 100 mL of the MB

solution by constantly agitating at 120 rpm until equilibrium was

reached. Following the adsorption stage, all the samples were

filtered through Fioroni 601 filter paper to make them carbon

free. The final concentration of MB was measured by a UV-Vis

spectrophotometer. The following equation was applied to

calculate the amount of MB adsorbed

qe=(Co−Ce)V

m

(4)

where Co and Ce are initial and final concentrations of MB at

equilibrium in mg L-1, respectively, m is mass of adsorbent in

gram and V is volume of solution in L.

2.6 Kinetic study and isotherm study

In the kinetic experiments, samples were collected at an interval

of 10 min for 3 hours and the amount of MB adsorbed was found

according to equation (2). An attempt was made to fit the

obtained adsorption data to equation (5) and (6) which are

standard linearized-integral form of pseudo first order and a

pseudo second order kinetic model respectively.

ln¿ (5)

where kl is the Lagergren rate constant of adsorption (min-1). The

values of qe and kl were determined from the plot of ln (qe − qt)

against t.

tqt

=1k2

1qe2 +

tqe

(6)

where k2 is the pseudo second-order rate constant of adsorption

(g mg-1 min-1). The values of the pseudo second-order rate

constants qe and k2 were calculated from the slopes and intercepts

of straight portion of the linear plots obtained by plotting t/qt

vs. t,

Isotherm study was conducted with the MB solution with initial

concentration of 50 to 250 mg L-1. The same amount of adsorbent

is added to 100 mL of solution of different concentrations. The

adsorption process was carried out in shake flasks for 24 hours

at 120 rpm at 27 oC temperature. In order to understand the

interaction between the metal ion and the adsorbent, two most

widely accepted mechanisms, namely Langmuir isotherm and

Freundlich isotherm were applied to model the experimental data.

The assumptions made in Langmuir isotherm are surface with

homogeneous binding sites and equivalent sorption energies having

no interaction between the adsorbed species. The linearized form

of the Langmuir equation can be expressed as follows

1qe

= 1qmax

+ 1CeqmaxKl

(7)

where qmax and Kl represent the maximum adsorption capacity for the

adsorbent and the energy constant related to the heat of

adsorption respectively. The values of qmax and b could be

calculated from the slopes and intercepts of the straight line

that was obtained by plotting 1/qe vs 1/Ce .

The Freundlich isotherm is an empirical equation based on an

exponential distribution of adsorption sites and energies. The

linear form of the equation is represented as follows

lnqe= lnKf+ 1n lnCe

(8)

where KF (L g-1) and n are Freundlich constants related to the

adsorption capacity and intensity, respectively. The calculations

of the Freundlich constants were obtained from the slope and

intercept of the straight line obtained by plotting ln qe against

ln Ce.

3. Results and discussions

3.1 Effects of the control factors and noise factor on the AC

preparation

Thirty two different activated carbon samples according to L16

array of Taguchi method and the noise factors were prepared and

tested for MB adsorption. The adsorption capacity of the

activated carbon and corresponding S/N ratio is given in Table 2.

The S/N ratio was tested by ANOVA to determine the significance

of the S/N data obtained. ANOVA determines the impact of the

independent variables on the dependent variables in a regression

analysis. ANOVA results of the S/N ratio is given in Table 3.

<Table 3>

Effects of control factors on the S/N ratio of the MB adsorption

of the prepared activated carbon can be observed in Fig. 1. Bold

values in Table 3 for the level averages are the maximum average

S/N performances of factors in the four different levels at each

factor. The largest S/N performance corresponds to the best

performance characteristic. According to Kirby (2006), the F-

ratio can indicate the effect of the control factor on the result

obtained. A F-ratio less than one suggests insignificant effect,

a value near about two suggests moderate effect and if the F-

ratio is more than four, the control factor has a strong and

significant effect. From Table 3, it can be observed that

microwave power, irradiation time have moderate effect on the

preparation with F-ratio 2.62 and 2.20 whereas impregnation ratio

clearly has very significant effect on the preparation of

activated carbon. Concentration of the acid had no such

significant impact on the quality of the activated carbon in

terms of MB adsorption. The S/N ratio for the microwave power

increased as the level increased from 400 W to 800 W, meaning

that the adsorption capacity increased. However, when the power

was higher than 800 W, the S/N ratio decreased, indicating the

decrease in adsorption capacity of the activated carbon. The

initial rise in the adsorption capacity may be attributed to the

formation of more pores and better pore structure. At highest

level of microwave power, the destruction of pores and burning of

the raw material to ash leading to the loss of adsorption sites

may have reduced the adsorption capacity (Deng et al., 2010).

Practically no change was observed in the S/N ratio for the first

two levels of irradiation time of 3 min and 10 min. The S/N ratio

and adsorption capacity increased steeply when the raw material

was heated for 17 min and decreased when heated for 24 min. The

increase in the adsorption capacity of the activated carbon at

the third level was due to the formation of the pores but at

higher level the excessive exposure to the microwave may have

damaged the pores and thereby the adsorption capacity is

decreased (Aber et al., 2009). The impregnation ratio found to

have a very significant effect on the adsorption capacity of the

activated carbon. At lower level of IR, the amount of acid was

too small to react with the amount of raw material adequately,

therefore the development of the micro and meso pores were not

facilitated. However, as the impregnation ratio increased the

cellular structure of palm kernel shells were attacked by the

acid which resulted in the rupture of the linkages between the

lignin and cellulose during impregnation stage followed by

formation of larger structural units and strong cross linked

solids (El-Hendawy et al., 2008). The concentration of the acid

have no significant effect on the adsorption capacity of the

activated carbon. However, it was found that the higher

concentration of acid had higher S/N ratio, probably because the

higher concentration of acid aided the bond breaking of the

lignin and cellulose.

<Fig. 1.>

3.2 Significance of noise factors

The significance of the effect of noise factor was analyzed from

t-test performed on the data set obtained for different noise

conditions. A two tailed t-test to examine the equality of means

assuming equal variances shows that the P-Value is 1.00 (Table 4)

which is much greater than 0.05; therefore, the null hypothesis

was accepted which implies that the means do not differ

significantly across the two samples. Consequently, we can

conclude that the noise factor for particle size had no

significant effect on the adsorption capacity of the activated

carbon.

<Table 4>

3.3 Optimization of the process variable and validation

The mean S/N ratio for each level of the control factors was

summarized as S/N response, which is shown in Table 3. The S/N

ratio should always be highest at the optimum condition, because

it is desired that the signal to be much higher than the noise.

Thus, the optimum condition for AC preparation corresponded to

the levels having largest average S/N ratio. Therefore, it can be

inferred that the optimum condition is the following: microwave

power of 800 W (level 3), irradiation time of 17 min (level 3),

impregnation ratio of 2 (level 4), and concentration of acid 85%

(undiluted). For further study a sample of activated carbon was

prepared using these settings of the control factors.

3.4 Characterization of the activated carbon prepared at optimum

condition

The characteristics of the activated carbon prepared at the

optimum condition was determined by N2 adsorption/desorption

curve with the aid of BET equation, SEM images and FTIR data.

The BET surface area was found to be 1,473.55 m² g-1. The

nitrogen adsorption/desorption curve as shown in Fig. 2 describes

a type IV curve as per the IUPAC guideline (Rouquerol et al.,

1994) representing adsorption isotherm with hysteresis mainly

associated with mesopores in the activated carbon. Though the

sharp “knee” shape (Hoseinzadeh Hesas et al., 2013) at the lower

relative pressure suggest a considerable amount of micro-pores

are present in the activated carbon. The average pore size was

found to be 2.78 nm which falls in the mesoporous range.

According to Liu et al., (2010) intense activation reaction

facilitated the formation of mesoporous activated carbon. Kubota

et al., (2009) also found similar mesoporous activated carbon and

attributed the formation of pores to the rapid and volumetric

heating, causing faster release of volatile matters from the raw

material.

<Fig. 2.>

The raw material had much less surface roughness and porosity on

the surface as evident from the SEM image given in Fig. 3A.

Activated carbon prepared in the optimum operating condition

developed considerable amount of porosity after activation, as

evident from Fig. 3B. The acid in the impregnation stage and

microwave heating in the activation stage contributed towards the

formation of pores and surface roughness. The FTIR data in Fig.

3C and D shows that the intensity of the peaks are between 3500-

3200 cm-1 which is attributed to O-H stretching of alcohols or

phenols, decrease to a great extent. Intensity of the band of

3000-2850 cm-1 corresponding to C-H stretch of alkanes has also

diminished in the activated carbon. Both phenomena indicate that

the phosphoric acid acted as dehydrating agent and removed a

considerable amount of hydrogen from the raw material (Hesas et

al., 2013) to conversion. Reduction of intensity of the complex

peaks around 1580-1650 cm-1 can be attributed to N-H bend of

primary amines while the C-C stretch of aromatic ring in

activated carbon is also observed, as can be seen from Fig. 3C

and D. The C-O stretch for alcohols, carboxylic acids, esters,

and ethers corresponding to the band 1300-1000 cm-1 had much lower

peak. These observations signified removal of hydrogen and oxygen

associated with the raw material.

<Fig. 3.>

3.5 Kinetic and Isotherm study of the AC prepared at optimum

condition

The data obtained from the kinetic experiments were validated

with a pseudo first order kinetic equation, popularly known as

Lagergren equation as well as pseudo second order kinetic

equation. The results obtained after regression analysis of the

data collected for pseudo first order and pseudo second order

kinetics are shown in Fig. 4A and 4B, respectively. The value of

regression coefficient R2 is used to select the rate equation

describing the adsorption process. The value of regression

coefficients and rate constants are tabulated in Table 5. The R2

value of pseudo second order equation were 0.9998 and 0.9974 for

solutions with initial concentrations of 100 mg L-1 and 200 mg L-

1, respectively. This observation suggests that pseudo second

order equation describes MB adsorption better than the pseudo

first order equation with R2 values of 0.8012 and 0.9213 for

solutions of initial concentrations of 100 mg L-1 and 200 mg L-1,

respectively. As the adsorption follows second order kinetics so

the chemisorption plays an important part in the rate limiting

step involving valence forces developed due to sharing or

exchange of electrons between adsorbent and adsorbate (Kushwaha

et al., 2008).

The isotherm parameter values enlisted in Table 5 clearly shows

that the adsorption follow Langmuir isotherm model based on the

higher value of regression coefficient R2. The dimensionless

parameter RL was calculated from the following Eq. 7.

RL= 11+KlC0

(7)

The value of RL suggests the favorability of adsorption. It may

be noted that RL > 1 suggests an unfavorable process, whereas

0<RL<1 suggests a favorable process. The RL values for initial

concentrations of 50 mg L-1, 100 mg L-1, 150 mg L-1, 200 mg L-1, and

250 mg L-1 were found to be 0.2857, 0.1667, 0.1177, 0.0910, and

0.0741, respectively, which suggests that the adsorption process

is favorable.

<Fig. 4.>

<Table 5>

4. Conclusion

The optimum condition suggested by the robust Taguchi Method for

AC preparation was microwave power of 800 W, irradiation time of

17 min, impregnation ratio of 2, and acid concentration of 85%.

The Noise factor (particle size) had no effect on AC quality and

under the optimum condition employed, the AC was highly porous

with surface area of 1,473.55 m² g-1. FTIR confirms dehydration

and removal of hydrogen from raw material. Langmuir Isotherm and

a pseudo-second-order kinetic model satisfactorily described

equilibrium and kinetic data, respectively. The maximum MB

adsorbing capacity was 1000 mg g-1.

Acknowledgements

The authors are grateful to University of Malaya (Project no.

UM.C/HIR/MOHE/ENG/13 and IPPP project no. PG040-2012B) for

providing the fund for the research work.

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Figure Captions

Fig. 1. The effect of control factors on the S/N ratio of the MB adsorption by the prepared AC samples.

Fig. 2. Nitrogen adsorption/desorption isotherm for the activatedcarbon prepared at the optimum condition showing type IV isotherm.

Fig. 3. SEM micrographs and FTIR spectra of the raw material (A and C) and prepared activated carbon at optimized condition (B and D) depicting surface characteristics.

Fig. 4. Regression analysis of the MB adsorption data for A) Pseudo-first-order kinetics and B) pseudo-second-order kinetics.

Table 1. Levels of the control factors used as preparation

parameters.

Investigatingparameters Level 1 Level 2 Level 3 Level 4

MicrowavePower (W) 400 600 800 1000

Time (min) 3 10 17 24Impregnation

ratio(acid:pks)

0.5 1 1.5 2

Conc. ofacid (H3PO4)

(%)42.5 85

Table 2. L16 experimental design with the noise factors and the measured value of MB adsorption and corresponding S/N ratios.

Inner control factor array Outer noisefactor array

Methylene blueadsorption (mg

g-1)

Experiment No

Microwave

Power(W)

Time(min)

Impregnation ratio(acid:pks

)

Conc.ofacid(H3PO4

) (%)

NF1(0.5-1mm)

NF2(1-2mm)

S/Nratio

1 400 3 0.5 42.5 7.46 19.4 19.872 400 10 1 42.5 28.15 30.1 29.263 400 17 1.5 85 100 100 404 400 24 2 85 100 100 405 600 3 1 85 84.43 81.59 38.376 600 10 0.5 85 18.85 41.26 27.697 600 17 2 42.5 100 100 408 600 24 1.5 42.5 100 87.6 39.389 800 3 1.5 85 100 67.74 37.9810 800 10 2 85 100 100 4011 800 17 0.5 42.5 77.81 63.26 38.5312 800 24 1 42.5 85.74 78.91 38.2813 1000 3 2 42.5 100 100 4014 1000 10 1.5 42.5 100 100 4015 1000 17 1 85 100 100 4016 1000 24 0.5 85 26.08 59.19 30.56

Table 3. ANOVA of the S/N ratios for the prepared activated

carbons.

Source DFa Seq

SSbAdjSSc

AdjMSd F P Level average

Level1

Level 2

Level 3

Level4

Microwavepower (W) 3 86.99 86.99 28.99 2.62 0.1

6 32.28 36.36

38.28 37.64

Irradiation

Time(min)3 72.82 72.82 24.27 2.20 0.2

0 34.06 34.24

39.21 37.06

Impregnation Ratio 3 320.2

0320.20

106.73 9.65 0.0

1 28.74 36.48

39.34 40.00

Conc ofAcid 1 7.55 7.55 7.55 0.68 0.4

5 35.45 36.83

ResidualError 5 55.28 55.28 11.06

Total 15 542.85

aDF: degree of freedom, bSeq SS: sequential sums of squares, cAdj SS: adjusted sum of squares, dAdj MS: Adjusted mean of squares.

Table 4. t-Test to find the significance of noise factors

 Variable 1

Variable 2

Mean 76.78 76.81

Variance1,205.8

8 748.99Observations 16.00 16.00Pooled Variance 977.44Hypothesized Mean Difference 0.00df 30.00t Stat 0.00P(T<=t) two-tail 1.00t Critical two-tail 2.04  

Table 5. Values of Isotherm Model Parameters

LangmuirIsotherm

FreundlichIsotherm Temkin Isotherm

qmax

(mg gm-

1)1000 Kf (L

g-1) 54.03 A (L g-

1) 30.27

Kl (Lg-1) 0.05 1/n 0.6905 b (kJ

mol-1) 138.88

n 1.45 B (Jmol-1) 18.13

R2 0.9614 R2 0.9154 R2 0.873